In the sewing of garment parts or other work products with a semi-automatic sewing machine that forms a overlock stitch in the garment parts a chain of thread is created which extends from the rear of each garment part back toward the sewing needle by the continuing operation of the sewing machine after the garment parts pass through the machine. This thread chain is formed as the sewing machine continues to operate for a short time after the garment part has passed through the sewing machine. The continued operation of the sewing machine after the garment part has passed the sewing needle is necessary to ensure that the stitching in the garment part has been completed and the thread chain extending off the garment part ensures that the stitching in the garment does not unravel. Typically, the trailing thread chain is severed after the garment part has moved beyond the sewing needles, leaving a tail of thread chain extending from the rear of the garment part, as well as leaving some excess thread chain extending from the sewing needle that will become attached to the next garment part that passes through the sewing machine.
Most semi-automatic, overlock stitch, sewing machines have one or more sewing needles for creating the thread chain, a presser foot, a throat plate, a chaining tongue, feed dogs, a motor, a foot treadle switch for engaging the sewing machine and other components and controls necessary for operation. The presser foot is used to hold the garment part down against the surface of the throat plate and against the cyclical motion of the feed dogs. The throat plate has slots through which feed dogs engage the garment part as the sewing action progresses, thereby advancing the garment part. The chaining tongue normally is formed as part of the throat plate, and acts as a knitting needle around which the thread chain is formed.
It has been known in the art to trim excess thread chain extending from the rear of the garment part with the use of a vacuum knife. In such known arrangements, a scissors-type cutter is arranged so that the knife blades are adjacent and parallel to the path of travel of the garment through the sewing machine. The knife blades are positioned in front of and to one side of an opening to a relatively large vacuum conduit (the diameter of the vacuum conduit is much greater than the thickness of the thread chain). As the garment part passes by the vacuum knife, the stream of air drawn through the vacuum conduit induces a length of the thread chain extending in a relaxed condition from the rear of the garment part to move into the vacuum conduit. The thread chain extending from the rear of the garment part is thereby extended across the knife blades and the blades trim the thread chain to an acceptably short length extending from the trailing edge of the just sewn garment part. This trimming of the thread chain also leaves a length of thread chain extending from the sewing needle and loops about the chaining tongue and into the sewing area of the sewing machine.
To prevent the excess thread chain that extends from the needles of the sewing machine from being attached haphazardly to the leading edge of the next oncoming garment part and spoiling the appearance of the next succeeding garment part as the garment is sewn, it is desirable to position this excess thread chain so that it will be oversewn or "latched back" into and substantially hidden by the stitching formed in the next garment part precisely at the beginning of the stitching in that garment part. In the absence of such a "latch back" operation, the excess thread chain creates a knot or chain of threads that extends from the leading edge of the subsequent garment part, and the excess thread chain may have to be trimmed away from the garment, which requires additional time and tends to weaken the stitched seam.
There are prior art disclosures which address the problem of positioning and oversewing the excess thread chain extending from the needles of the sewing machine into the subsequent garment part. For example, U.S. Pat. No. 4,679,515 of Keeton describes an apparatus which works to oversew the excess thread chain by severing the thread chain, blowing the free end of the thread chain extending from the sewing needles around and away from the sewing area with a stream of air, and snagging the excess thread chain with a mechanical finger or other means, thereby holding the thread chain in such a position that it should be oversewn into the next garment part. However, there are certain practical difficulties with this type arrangement, most notably the difficulty in correctly positioning the beginning of the excess thread chain at the very beginning of the next garment part. Another shortcoming of the prior art is that the mechanical device does not always snag the excess thread chain successfully, making the apparatus inefficient to use in a high production environment. Such prior art further requires a relatively large number of moving parts to perform the latch back sewing function, while generally a device which has fewer moving parts tends be more reliable and simpler to construct and maintain.
U.S. Pat. No. 4,038,933 of Marforio discloses a sewing machine with a vacuum operated device for latching back excess thread chain at the beginning of forming a chain stitch in a garment. Marforio discloses a vacuum knife for severing the thread chain and a hollow chaining tongue for drawing therein the severed thread chain extending from the needles by applying a vacuum to the hollow chaining tongue. Devices made according to the teachings of Marforio generally fail to latch back thread chains into the chain stitch of the next garment with a high degree of reliability at least in part because Marforio does not disclose a means for ensuring that the thread chain that would be drawn into the hollow chaining tongue is of a predetermined length. Applicant has discovered that if the thread chain to be drawn into the hollow chaining tongue is too long, it cannot be drawn into the hollow chaining tongue reliably. Also, if the thread chain to be drawn into the hollow chaining tongue is too short, it may be too stiff to respond to the stream of air that attempts to turn the thread chain toward the chaining tongue and the short thread chain can become unraveled, producing a weakened and unattractive latch back stitch. Marforio discloses using a Venturi air flow inducement constriction to create a source of reduced pressure for drawing the severed thread chain with an air stream into the hollow chaining tongue. Such an arrangement generally is not desirable because this type system usually creates a loud noise in operation from consuming large volumes of compressed air to create the desired air stream and from a failure to produce a large enough volume of air flow through the hollow chaining tongue to move the excess thread chain into the hollow chaining tongue.
Thus, it is seen that a need exists for a sewing machine assembly which will "latch back" leftover thread chain severed from a previously sewn garment part and extending from the sewing needles precisely at the beginning of the stitching of a second garment part, while maintaining a high degree of reliability of operation.